There are really 2 questions here. So lets deal with each of them:
1. Should I display a cookie notice?
The answer to that is not really opinion based, its a legal issue. If you run a website that is owned in the EU you must make sure that your website complies with the latest "cookie laws" - The law is fairly detailed, but the basics say that:
source: The Cookie Law
It is important to note, that if the user declines cookies - you must not set cookie values on your site - the user should not have to turn cookies off in the browser to decline them.
Another important thing to note is that only certain tracking cookies can be declined. Cookies that are essential to the "business" use of the site, can still be stored. E.g. forms auth cookies.
2. What will Sitecore track/do if cookies are disabled
Just because a cookie is declined or disabled, Sitecore will still track the user in a single session. Declining or disabling cookies does not require you to disable the tracker.
What you will lose is the ability to track anonymous users from session to session. You will not get a global session for that user.